Aluminum members are increasingly used as components of a vehicle such as an automobile or a motorcycle to reduce the weight of the vehicle. Aluminum members are widely used particularly in a heat exchanger incorporated in a vehicle. Connection of aluminum members, for example, an aluminum tube and an aluminum connector or union is usually carried out by a braking method as described below: Aluminum members to be joined are tacked by means of an appropriate jig. Then, the joint of the thus tacked aluminum members is covered with flux as required. A lumpy, wire-shaped, bar-shaped, strip-shaped, or powdery filler metal, or a filler metal previously formed into the shape matching with that of the joint is added to the joint, and the joint is heated with a torch to melt the filler metal. The joint is thus brazed by the melted filler metal. Methods are also available which comprise conducting brazing by heating the aluminum members added with the filler metal and flux on the joint in a furnace, and which comprise applying shock by ultrasonic wave to the joint and carrying out brazing while removing an oxide film formed on the surfaces of the filler metal and the aluminum members by this shock.
As the above-mentioned filler metal for brazing the aluminum members, there is conventionally employed an Al-Si-base filler metal containing from 9 to 13 wt. % Si as denominated BA4343, BA4045 or BA4047 in the JIS (Japanese Industrial Standards). The Al-Si-base filler metal has a solidus temperature of 577.degree. C. and a liquidus temperature of from 577.degree. to 620.degree. C. The brazing temperature range of Al-Si-Base filler metal, i.e., the temperature range within which an appropriate fluidity for brazing is produced in the filler metal is close to the solidus temperature of the aluminum members to be joined. As a result, the portion near the joint of the aluminum members is melted or deformed unless the brazing temperature is strictly controlled.
On the other hand, the above-mentioned connection of aluminum members may be accomplished also by a soldering method which comprises: using a Zn-base solder comprising 100 wt. % Zn or comprising 95 wt. % Zn and 5 wt. % Al, and soldering a joint of aluminum members by means of the solder in accordance with the same method as in the brazing method described above. The Zn-base solder has a soldering temperature range far lower than the solidus temperature of the aluminum members. The problems as posed in the above-mentioned Al-Si-base filler metal are non-existent in the solder. However, the Zn-base solder has the following problem: A tough oxide film is formed on the surface of the Zn-base solder. When carrying out soldering, therefore, it is necessary to destroy the oxide film produced on the solder surface by strongly rubbing the joint of the aluminum members with the solder, thus resulting in many troubles in the soldering operation. Insufficient destruction of the oxide film prevents formation of a satisfactory fillet on the joint and causes deterioration of the quality of the joint.
With a view to solving the above-mentioned problems, the following filler metal suitable for brazing aluminum members is proposed:
A filler metal for brazing aluminum members, as disclosed in Japanese Patent Publication No.50-35,018 dated Nov. 13, 1975, which consists essentially of:
The above-mentioned prior art has the following problem: Since the zinc content is as low as within the range of from 4 to 10 wt. %, this filler metal has a brazing temperature range close to the solidus temperature of the aluminum members. It is therefore necessary to conduct brazing at a high temperature near the solidus temperature of the aluminum members, thus resulting in melting or deformation of the portion near the joint of the aluminum members.
Under such circumstances, there is a strong demand for development of a filler metal for brazing aluminum members, which permits brazing of aluminum members at a temperature lower than the solidus temperature of the aluminum members, gives a satisfactory fluidity of the melted filler metal, allows, during brazing, easy destruction of an oxide film formed on the surface of the filler metal, and thus leads to a joint excellent in quality, but a filler metal having such properties has not as yet been proposed.